1. Field of the Invention
The present invention relates to an aluminum alloy piping material. More particularly, the present invention relates to an aluminum alloy piping material having an excellent corrosion resistance and workability, which is suitably used for pipes connecting automotive radiators and heaters or pipes connecting evaporators, condensers, and compressors.
2. Description of Background Art
Pipes used for passages connecting automotive radiators and heaters or passages connecting evaporators, condensers, and compressors are provided with a bulge at the pipe ends, and connected to radiators, heaters, evaporators, condensers, or compressors. Pipes connected to radiators or the like are connected to a rubber hose and fastened using a metal band. As the piping material, a single pipe consisting of an Alxe2x80x94Mn alloy such as a 3003 alloy, and a two-layered or three-layered clad pipe consisting of an Alxe2x80x94Mn alloy as a core material and an Alxe2x80x94Zn alloy sacrificial anode material such as a 7072 alloy clad on the core material have conventionally been used.
In the case where the Alxe2x80x94Mn alloy piping material is used under a severe environment, pitting corrosion or intergranular corrosion tends to occur. When the Alxe2x80x94Mn alloy piping material is connected to a rubber hose, crevice corrosion occurs in the piping material at the inner side of the rubber hose, specifically, the outer side of the piping material. Use of a clad pipe prevents the occurrence of pitting corrosion or crevice corrosion. However, this significantly increases costs.
In order to solve the above problem, Japanese Patent Application Laid-open No. 4-285139 proposes a piping material exhibiting improved crevice corrosion resistance, in which Cu and Ti are added to an Alxe2x80x94Mn alloy and the Fe content and the Si content are limited within a specific range. This piping material has good properties under various use conditions. However, this piping material may exhibit insufficient workability during bulge formation of the pipe ends when used as a pipe. Moreover, this piping material has a problem with corrosion resistance when allowed to stand under a severe corrosive environment.
The present inventors have examined the above problems of the Alxe2x80x94Mn alloy piping material relating to a decrease in workability and corrosion resistance. As a result, the present inventors have found that a decrease in the corrosion resistance is caused by microgalvanic corrosion occurring between an alloy matrix and various intermetallic compounds present in the matrix. The present inventors have also found that workability at the pipe ends is affected by the distribution state of the intermetallic compounds.
The present invention has been achieved as a result of further experiments and studies on the Alxe2x80x94Mn alloy piping material based on the above findings. Accordingly, an object of the present invention is to provide an aluminum alloy piping material exhibiting good corrosion resistance even under a severe corrosive environment and having an excellent workability, such as bulge formation capability at the pipe ends.
One aspect of the present invention provides an aluminum alloy piping material having an excellent corrosion resistance and workability, comprising an aluminum alloy which comprises 0.3-1.5% of Mn, 0.20% or less of Cu, 0.06-0.30% of Ti, 0.01-0.20% of Fe, and 0.01-0.20% of Si, with the balance consisting of Al and impurities, wherein, among Si compounds, Fe compounds, and Mn compounds present in the matrix, the number of compounds with a particle diameter of 0.5 xcexcm or more is 2xc3x9710 4 or less per mm2.
In this aluminum alloy piping material having an excellent corrosion resistance and workability, the aluminum alloy may further comprise 0.4% or less of Mg.
In the above aluminum alloy piping material having an excellent corrosion resistance and workability, the aluminum alloy may further comprise at least one of 0.01-0.2% of Cr and 0.01-0.2% of Zr.
In the above aluminum alloy piping material having an excellent corrosion resistance and workability, the Cu content in the aluminum alloy may be 0.05-0.10%.
In the above aluminum alloy piping material having an excellent corrosion resistance and workability, the Fe content in the aluminum alloy may be 0.01-0.09%.
In the above aluminum alloy piping material having an excellent corrosion resistance and workability, the number of compounds with a particle diameter of 0.5 pm or more may be from 1xc3x97203 to 2xc3x97104 per mm2.
In the above aluminum alloy piping material having an excellent corrosion resistance and workability, the tensile strength of the softened material (O material) may be 130 MPa or less.
The effects of alloy components of an aluminum alloy piping material having an excellent corrosion resistance and workability of the present invention, and reasons for the limitations of the aluminum alloy are described below. Mn increases the strength and improves corrosion resistance, in particular, pitting corrosion resistance. The Mn content is preferably 0.3-1.5%. If the Mn content is less than 0.3%, the effect may be insufficient. If the Mn content exceeds 1.5%, a large number of Mn compound particles may be formed, whereby the corrosion resistance may decrease. The Mn content is still more preferably from 0.8% or more to less than 1.2%.
Cu improves the strength of the alloy. The Cu content is preferably 0.20% or less. If the Cu content exceeds 0.20% corrosion resistance may decrease. The Cu content is still more preferably 0.05-0.10%.
Ti is separately distributed in a high-concentration area and a low-concentration area. These areas are alternately layered in the direction of the thickness. The low-concentration area is preferentially corroded rather than the high-concentration area, thereby forming corrosion layers. This prevents the corrosion from proceeding in the direction of the thickness, thereby improving pitting corrosion resistance, intergranular corrosion resistance, and crevice corrosion resistance of the material. The Ti content is preferably 0.06-0.30%. If the Ti content is less than 0.06%, the effect may be insufficient. If the Ti content exceeds 0.30%, giant compounds may be produced during casting, thereby decreasing workability. As a result, a sound piping material cannot be obtained. The Ti content is still more preferably 0.15-0.25%.
Fe decreases the grain size after extrusion or the grain size after drawing and annealing, thereby improving formability of the piping material. This prevents the occurrence of cracks or surface roughening during bulge formation or the like. The Fe content is preferably 0.01-0.20%. If the Fe content is less than 0.01%, the effect may be insufficient. If the Fe content exceeds 0.20%, a large number of Fe compound particles may be formed, whereby the corrosion resistance may decrease. The Fe content is still more preferably 0.01-0.09%.
Si decreases the grain size after extrusion or the grain size after drawing and annealing in the same manner as Fe, thereby improving formability of the piping material. This prevents the occurrence of cracks or surface roughening during bulge formation or the like. Moreover, Si forms Alxe2x80x94Mnxe2x80x94Si compounds and Alxe2x80x94Mnxe2x80x94Fexe2x80x94Si compounds, thereby preventing the occurrence of penetration between tools and the material during bending, bulge formation, or the like. The Si content is preferably 0.01-0.20%. If the Si content is less than 0.01%, the effect may be insufficient. If the Si content exceeds 0.20%, a large number of Si compound particles may be formed, whereby the corrosion resistance may decrease. The Si content is still more preferably 0.01-0.10%.
Mg increases the strength and decreases the grain size. The Mg content is preferably 0.4% or less (but more than 0%). If the Mg content exceeds 0.4%, extrusion capability and corrosion resistance may decrease. The Mg content is still more preferably 0.20% or less.
Cr and Zr are separately distributed in a high-concentration area and a low-concentration area in the same manner as Ti. These areas are alternately layered in the direction of the thickness. The low-concentration area is preferentially corroded rather than the high-concentration area, thereby forming corrosion layers. This prevents corrosion from proceeding in the direction of the thickness, thereby improving pitting corrosion resistance, intergranular corrosion resistance, and crevice corrosion resistance of the material. The Cr content and the Zr content are preferably 0.01-0.2% and 0.01-0.2%, respectively. If the Cr or Zr content is less than the lower limit, the effect may be insufficient. If the Cr or Zr content exceeds the upper limit, giant compounds are produced during casting, thereby decreasing workability. As a result, a sound piping material cannot be obtained.
Depending upon the distribution of Si compounds (compounds containing Si such as Alxe2x80x94Si compounds), Fe compounds (compounds containing Fe such as Alxe2x80x94Fe compounds such as FeAl3 and Alxe2x80x94Fexe2x80x94Si compounds such as xcex1-AlFeSi), and Mn compounds (compounds containing Mn such as Alxe2x80x94Mn compounds such as Al6Mn, Alxe2x80x94Mnxe2x80x94Si compounds such as Mn3SiAl2, and Alxe2x80x94Mnxe2x80x94Fexe2x80x94Si compounds such as xcex1-AlMnFeSi), which are distributed in the alloy matrix, microgalvanic corrosion may occur between the compound particles and the matrix. In order to increase the corrosion resistance by preventing the occurrence of microgalvanic corrosion, it is important to limit the number of compounds with a particle diameter (equivalent circle diameter) of 0.5 pm or more among Si compounds, Fe compounds, and Mn compound within 2xc3x97104 or less per mm2.
The number of the above compounds is preferably from 1xc3x9710 3 to 2xc3x97104 per mm2. Such a distribution improves corrosion resistance and improves workability due to an increased elongation. The number of the above compounds is still more preferably from 1xc3x97103 to 1xc3x97104 per mm2.
The aluminum alloy piping material according to the present invention is produced by casting a molten metal of an aluminum alloy having the above composition into a billet by continuous casting (semicontinuous casting); homogenizing the resulting billet; hot extruding the billet into the shape of a pipe; and annealing the formed product. In addition, the billet formed into the shape of a pipe by hot extrusion may be further drawn before annealing.
The above distribution of Si compounds, Fe compounds, and Mn compounds is obtained by adjusting the cooling rate during continuous casting and the homogenization conditions for the billet. For example, the above distribution of the Si compounds, Fe compounds, and Mn compounds is obtained by decreasing the surface level of the molten metal in a mold during continuous casting to half or less of the usual level, or increasing the casting rate from 1.2 to 1.3 times the usual rate. It is preferable to perform homogenization at a temperature of 600xc2x0 C. or more. Workability can be improved by allowing the tensile strength of the softened material (O material) after annealing to be 100-130 MPa, whereby bulge formation or the like becomes easy.